(1) Field of the Invention
The present invention pertains to an epicyclic gear transmission that is primarily intended for use in rotary wing aircraft, i.e., helicopters. More specifically, the present invention pertains to a high ratio, double helical epicyclic gear transmission that employs double helical planet gears to obtain a reduction in size of the transmission, to improve the safety of the transmission, and to reduce the noise created by the operation of the transmission.
(2) Description of the Related Art
In rotary wing aircraft, the main rotor gear transmission is the most critical and usually the heaviest single subsystem in the drive system of the aircraft. This is true for a single or tandem rotor helicopter, or a tilt rotor aircraft.
Currently, the final transmission stage in virtually all main rotor drives is an epicyclic transmission system that typically consist of one or two simple, spur gear planetary stages. These planetary gear stages are composed of a sun gear that is driven by the input shaft of the transmission, multiple planet gears (typically between three and six) that intermesh with the sun gear and are spacially arranged around the periphery of the sun gear, and an orbit gear or internal ring gear that intermeshes with and surrounds the multiple planet gears. The orbit or internal ring gear is generally the fixed member of the epicyclic transmission system. The multiple planet gears are mounted to a carrier which in turn is operatively connected to the output shaft of the transmission. While this basic epicyclic gear transmission provides relatively good power efficiency, it has a tendency to generate high noise levels in operation due to the limited contact ratio of the sun gear and the internal ring gear with the multiple planet gears.
In addition to the undesirable high noise levels generated by the basic epicyclic gear transmission, the load capacity of the epicyclic gear transmission is limited by it being a function of the number of planet gears that can be accommodated inside the internal ring gear. The size of the internal ring gear itself is dependent on the combination of gear ratio requirements and the allowable stress limits of the transmission system. The planet gear bearings, which are typically mounted inside the center bore of each planet gear, must be sized to carry the loads applied to the planet gears and also to allow a minimum thickness of the planet gear rim inside the planet gear teeth that surrounds the planet gear bearings. This requires that the size of the planet gear and the size of the planet gear bearings be optimized as a unit.
Because the gears of the basic epicyclic gear transmission are densely packed to reduce the size of the transmission, a gear tooth failure of one of the gears can generally be very destructive. The high power density provided by an epicyclic gear transmission and the very limited space among the gears of the transmission causes a great deal of consequential damage from a fractured tooth in the transmission. While it would appear that the construction of the epicyclic gear transmission with its multiple planet gears meshing between the sun gear and internal ring gear would provide parallel load paths, which might offer some redundancy in transmission paths through the transmission, the opposite is true. It has been observed that the planet gears themselves provide little in the way of fail safety. The dense positioning of the multiple planet gears between the sun gear and internal ring gear almost assures consequential damage will occur in the transmission due to a gear tooth failure. This is especially true when one considers that once a planet gear loses its load-transmitting capability, the internal load balance between the multiple planet gears is compromised so that the planet gears no longer balance their own radial loads on the sun gear and the internal ring gear.
Despite the limitations of epicyclic gear transmissions cited above, the use of epicyclic gear transmissions in rotary wing aircraft is still desirable for their ability to provide relatively large reduction ratios in a compact package. What is needed to overcome the disadvantages associated with the basic epicyclic gear transmission are design enhancements to the basic design of the transmission that will reduce the weight of the transmission, provide a smaller footprint of the transmission, improve transmission safety and the ability to withstand single gear tooth failures, while decreasing the generated noise level of the transmission, all of which are required for rotary wing aircraft applications.